Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale. Recently, J-G. Zhu et al. described another spintronic device called a spin transfer oscillator in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry.
Both MRAM and STT-MRAM may have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. In another aspect, the MTJ may be based on a GMR effect where a reference layer and free layer are separated by a metal spacer. The MTJ element is typically formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line at locations where the top electrode crosses over the bottom electrode. A MTJ stack of layers may have a bottom spin valve configuration in which a seed layer, an anti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer, a thin tunnel barrier layer, a ferromagnetic “free” layer, and a capping layer are sequentially formed on a bottom electrode. The AFM layer holds the magnetic moment of the pinned layer in a fixed direction. The pinned or reference layer has a magnetic moment that is fixed in the “y” direction, for example, by exchange coupling with the adjacent AFM layer that is also magnetized in the “y” direction. The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the pinned layer. The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current is passed from the top electrode to the bottom electrode in a direction perpendicular to the MTJ layers, a lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“0” memory state) and a higher resistance is noted when they are in an anti-parallel state or “1” memory state.
In a read operation, the information stored in a MRAM cell is read by sensing the magnetic state (resistance level) of the MTJ element through a sense current flowing top to bottom through the cell in a current perpendicular to plane (CPP) configuration. During a write operation, information is written to the MRAM cell by changing the magnetic state in the free layer to an appropriate one by generating external magnetic fields as a result of applying bit line and word line currents in two crossing conductive lines, either above or below the MTJ element. One line (bit line) provides the field parallel to the easy axis of the bit while another line (digit line) provides the perpendicular (hard axis) component of the field. The intersection of the lines generates a peak field that is engineered to be just over the switching threshold of the MTJ.
A high performance MRAM MTJ element is characterized by a high tunneling magnetoresistive (TMR) ratio which is dR/R where R is the minimum resistance of the MTJ element and dR is the change in resistance observed by changing the magnetic state of the free layer. A high TMR ratio and resistance uniformity (Rp_cov), and a low switching field (Hc) and low magnetostriction (λs) value are desirable for conventional MRAM applications. For Spin-MRAM (STT-MRAM), a high λs and high Hc leads to high anisotropy for greater thermal stability. RA should be relatively small (about 4000 ohm-μm2 or less) for MTJs that have an area defined by an easy axis and hard axis dimensions of less than 1 micron.
As the size of MRAM cells decreases, the use of external magnetic fields generated by current carrying lines to switch the magnetic moment direction becomes problematic. One of the keys to manufacturability of ultra-high density MRAMs is to provide a robust magnetic switching margin by eliminating the half-select disturb issue. For this reason, a new type of device called a spin transfer (spin torque) device was developed. Compared with conventional MRAM, spin-transfer torque or STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a CPP configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high, and if the dimensions of the multilayer are small. The difference between a STT-MRAM and a conventional MRAM is only in the write operation mechanism. The read mechanism is the same.
Referring to FIG. 1a, a conventional STT-MRAM structure 1 is shown and includes a gate 5 formed above a p-type semiconductor substrate 2, a source 3, drain 4, word line (WL) 6, bottom electrode (BE) 7, and bit line (BL) 9. There is also a MTJ element 8 formed between the bit line 9 and bottom electrode 7, and a via 10 for connecting the BE to the drain 4.
Materials with PMA are of particular importance for magnetic and magnetic-optic recording applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement and have a low switching current density but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is one of the key challenges for future MRAM applications and other spintronic devices. Theoretical expressions predict that perpendicular magnetic devices have the potential to achieve a switching current lower than that of in-plane magnetic devices with the same magnetic anisotropy field according to S. Mangin et al. in Nat. Mater. 5, 210 (2006).
When the size of a memory cell is reduced, much larger magnetic anisotropy is required because the thermal stability factor is proportional to the volume of the memory cell. Generally, PMA materials have magnetic anisotropy larger than that of conventional in-plane soft magnetic materials such as NiFe or CoFeB. Thus, magnetic devices with PMA are advantageous for achieving a low switching current and high thermal stability. Several groups have studied spin transfer switching in GMR multilayers with perpendicular magnetic anisotropy and reported their results including the aforementioned S. Mangin publication as well as H. Meng et al. in “Low critical current for spin transfer in magnetic tunnel junctions”, J. Appl. Phys. 99, 08G519 (2006), X. Jiang et al. in “Temperature dependence of current-induced magnetization switching in spin valves with a ferromagnetic CoGd free layer”, Phys. Rev. Lett. 97, 217202 (2006), T. Seki et al. in “Spin-polarized current-induced magnetization reversal in perpendicularly magnetized L10-FePt layers”, Appl. Phys. Lett. 88, 172504 (2006), and S. Mangin et al. in “Reducing the critical current for spin-transfer switching of perpendicularly magnetized nanomagnets”, Appl. Phys. Lett. 94, 012502 (2009). However, in the GMR devices described in the prior art, typical switching current density is above 10 milli-amp/cm2 which is too high for low switching current MRAM. Furthermore, the MR ratio is around 1% which is too small for the readout signal in MRAM. Therefore, improving the spin transfer switching performance in MTJ elements with PMA is extremely important for high performance MRAM applications.
There is a report by M. Nakayama et al. in “Spin transfer switching in TbCoFe/CoFeB/MgO/CoFeB/TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy”, J. Appl. Phys. 103, 07A710 (2008) on spin transfer switching in a MTJ employing a TbCoFe PMA structure. However, in a MTJ with a TbCoFe or FePt PMA layer, strenuous annealing conditions are usually required to achieve an acceptably high PMA value. Unfortunately, high temperatures are not so practical for device integration.
PMA materials have been considered for microwave assisted magnetic recording (MAMR) as described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magn., Vol. 44, No. 1, pp. 125-131 (2008). A mechanism is proposed for recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. FIG. 1b is taken from the aforementioned reference and shows an ac field assisted perpendicular head design. The upper caption 19 represents a perpendicular spin torque driven oscillator for generating a localized ac field in a microwave frequency regime and includes a bottom electrode 11a, top electrode 11b, perpendicular magnetized reference layer 12 (spin injection layer), metallic spacer 13, and oscillating stack 14. Oscillator stack 14 is made of a field generation layer 14a and a layer with perpendicular anisotropy 14b having an easy axis 14c. The ac field generator in the upper caption 19 is rotated 90 degrees with respect to the lower part of the drawing where the device is positioned between a write pole 17 and a trailing shield 18. The writer moves across the surface of a magnetic media 16 that has a soft underlayer 15. The reference layer 12 provides for spin polarization of injected current (I). Layers 14a, 14b are ferromagnetically exchanged coupled. Improved materials for the reference layer and oscillator stack are needed as this technology matures.
In other prior art references, U.S. Pat. No. 7,495,434 discloses a Ta and/or Ru seed layer and a sensor layer made of CoFeNi. U.S. Patent Application 2007/0278602, U.S. Patent Application 2008/0273380, and U.S. Patent Application 2009/0166322 describe a CoFeNi reference or free layer. U.S. Patent Application 2008/0070063 discloses a Ru—Rh seed layer and a reference layer comprising at least one of Fe, Ni, and Co.
Ta and Ti have been employed as a buffer layer or stress reduction layer in magnetic media as described in U.S. Pat. No. 6,911,256. U.S. Pat. No. 7,494,726 discloses a magnetic recording layer comprised of CoFeNi with a seed layer of Ni based alloy and Ti and Ta. However, none of the aforementioned references discuss a laminated free layer or reference layer with perpendicular magnetic anisotropy.